When delimbing tree trunks using a tree processing assembly, it is desirable to cut the branches as close as possible to the trunk. During a delimbing operation it is therefore necessary to adapt the grip of the surrounding knives to the varying thickness of the trunk section that is currently in the grip.
In a prior art delimbing device of the above mentioned type and disclosed in U.S. Pat. No. 4,898,218, the compliance of the gripping state is predetermined by changing a closing extent of a gripping means provided with the limbing knives each time a top face of the trunk has been moved a predetermined distance in a direction of movement for a resiliently compliant limbing knife. In the example disclosed in the above patent, there is provided a switch which is actuated by the compliant limbing knife so as to when the switch is not actuated by the limbing knife, the closing extent of the gripping means increases to lift the tree trunk in the delimbing device until the compliant limbing knife actuates the switch that then interrupts the closing of the gripping means. When the tree trunk advances further in the delimbing device in the direction to its top end, the switch again becomes non-actuated by the returning resiliently compliant limbing knife such that the closing extent of the gripping means once more increases. This procedure is repeated until the whole tree trunk has been advanced through the delimbing device.
A disadvantage of the prior art device is that the compliance of the gripping state, i.e. the remaining free stroke of travel of the resilient limbing knife, is invariable after each increase of the extent of closing of the grip of the trunk. However, in order to obtain a good delimbing operation, it may be necessary to cut the branches of different types of tree trunks with mutually different compliance or remaining free stroke of travel of the resilient limbing knife. Birch trees, for example, can have coarse branches that may excessively load the processing assembly when attempting to cut the branches too close to the trunk. The gripping state should then have a relatively large compliance, capable of allowing the tree trunk to move away from the limbing knives, or allowing the limbing knives to move away from the tree trunk a further distance, when one or more such coarse branches are coming into knife engagement. The compliance, play or remaining free stroke of travel of the gripping state should then be about 30 to 40 mm. On the other hand, spruce trees, for example, may have a linearly tapering trunk with relatively thin branches. The gripping state should then have a relatively small compliance so as to cut the branches close to the trunk. If the compliance then is too large, the elastic branches may resiliently bend toward the trunk and slide along the limbing knives without being cut. The compliance, play or free stroke of travel of the gripping state may then be about 5 mm.